Temporary regression after upgrading to 2.1.X or 2.2.X

Hi,
This regression can happen when:
- Upgrading from any 2.1 version (till 2.1.15) to latest 2.1.20
- Upgrading from any 2.2 version (till 2.2.7) to latest 2.2.12
- And having a descending clustering key
- And using CAS
Note that once SSTables are upgraded, the problem disappears.
A ticket has been opened here:
https://issues.apache.org/jira/browse/CASSANDRA-14462
And a patch has been proposed (quite a simple one).
I'd be glad if someone would like to review/help.
Thanks
Fabien